The present invention relates to an apparatus for treating fiber suspension. Said apparatus is especially applicable for screening and also thickening pulp in the wood processing industry. More precisely the inventive concept of the apparatus of the invention is related to the pulse member construction of the screen or thickener being used.
There are in principle two known types of pulse members, which both are generally used and the purpose of which is, as known, to maintain the screen surface clear, in other words, to prevent the formation of a fiber matting on the screen surface. One type is exemplified, for example, by a rotor in accordance with U.S. Pat. No. 4,193,865, in which said rotor is rotatably mounted within a stationary screen cylinder, comprising blades located close to the surface of the screen cylinder. In accordance with said patent the blades form an angle with the axis of the cylinder. The screen surface is subjected to pressure pulses by the moving blades clearing the openings of the screen surface. There are also embodiments that have blades on both sides of the screen cylinder. Consequently, the suspension being treated is fed either to the inside or to the outside of the cylinder and the accept is discharged from the outside or from the inside of the cylinder, respectively.
Another rotor type worth mentioning is, for example, one in accordance with U.S. Pat. No. 3,437,204, in which the rotor is a substantially cylindrical closed body, the surface of which is provided with almost hemispherical protrusions, so called bulges. In said apparatus the pulp is fed to a treatment space between the rotor cylinder and the screen cylinder outside thereof, whereby the protrusions of the rotor, the bulges, act both to press the pulp against the screen cylinder with the leading surface and to draw off the fiber flocs from the perforations of the screen cylinder with its trailing surface. Because this kind of construction has a highly thickening effect on the pulp, there are in the above mentioned arrangement three dilution water connections arranged at different heights on the screen cylinder, so as to make the screening of fiber suspension take place satisfactorily. A corresponding "bump rotor" is disclosed also in U.S. Pat. No. 3,363,759.
Also, other embodiments of the above mentioned cylindrical rotor are known, in connection with which many kinds of protrusions facing the screen cylinder may be used as disclosed in various publications.
DE application 30 06 482 discloses a knot separator, in which on the surface of a cylindrical rotor there are plough-like protrusions, made of plate material, by which the pulp between the rotor and the screen cylinder is subjected to strong mixing forces so as to make fibers pass through the screen cylinder most effectively, and knots, shives and such separate therefrom.
U.S. Pat. No. 3,400,820 discloses a rotor being formed of eccentric portions or lobes. The slope of the fluid contacting surface of the rotor is gradual in the increasing as well as decreasing clearance regions. The slope angle is less than 15.degree. relative to the direction of the movement. Since the slope is equal in both directions the operation of the rotor is far from optimal. The decreasing clearance presses the pulp towards the screen surface such that the openings of the screen cylinder easily clog, whereby the portion having the increasing clearance is not able to create a negative pressure sufficient to draw the fiber agglomerations out of the openings. Yet another drawback of the structure shown in the patent is, on the one hand, its tendency to crush all the impurities like knots etc. between the rotor and the screen surface. On the other hand, if the impurities are hard, such as stone or metal particles, there is a risk that the rotor presses the particles through the screen cylinder.
U.S. Pat. Nos. 4,188,286 and 4,202,761 disclose a screen apparatus, in which there is a rotatable cylindrical rotor inside the screen cylinder. There are protrusions arranged on the rotor facing the screen cylinder side, which protrusions have a V-shaped axial cross section so that on one rotational edge of the rotor there is a surface coming closest to the screen cylinder and being parallel to the rim of the rotor, and having an end surface substantially perpendicular to the surface of the rotor. These protrusions are arranged on the surface of the rotor cylinder axially in a certain angle position so that all protrusions of the rotor are in the same disposition with respect to the shaft of the rotor.
According to the prior art publications pulp can be fed to this apparatus to either side of the screen cylinder. If pulp is fed to the outside of the screen cylinder and accept is discharged from the interior of the screen cylinder, in other words from the rotor side, the rotational direction of the rotor is such that the accept is subjected by the angle position of the protrusions to a force component directed downwards and that said inclined/ascending surface functions as a front surface. If, however, pulp is fed between the rotor and the screen cylinder, in other words the accept is discharged from exterior of the screen cylinder, the rotational direction is opposite to the former. The protrusions tend to slow down the downward pulp flow and the surface upright to the surface of the rotor cylinder functions as a front surface.
Practical experience in the industry has, however, proved that the above mentioned arrangements do not operate satisfactorily in all circumstances. The apparatus also tends to dilute the accept and is therefore not applicable in cases where pulp with constant consistency is needed. Because the foils in the foil rotors are considerably far apart (4 to 8 foils), fiber matting will always be formed on the screen cylinder before the next foil wipes it off. Thus the use of a screen is not efficient. Moreover, said rotor type is expensive to manufacture because of the accurate dimensioning requirements of the rotor and the careful finishing of it. The ability of the foil to wipe off the fiber suspension, which accumulates on the surface of the screen cylinder is relatively poor. Further, the conventional foil resembling a wing of an aeroplane of its cross-section causes problems in the screening. It has been noted that the curved front edge, which is flow-dynamically appropriately designed causes the suspension to be treated to clog between the foil and the screen surface, in other words it causes a strong pressure stroke, due to which also impurities, such as shives tend to penetrate the openings of the screen surface. In order to be able to draw off such shives which are pressed with high pressure to the openings, a very strong negative pressure pulse is required.
It is typical of all above described pulse members that when aiming at the increase of the screening capacity, for example, by using higher pressure differences or when aiming at the increase of the purity of the accept by diminishing the size of the perforation or slot one comes to a point where the screen cylinder tends to clog. The problem is thus maintaining the screen cylinder clear, the ensuring of which offers the opportunity to optimize other factors connected to the screening, which, however, the following patent publication well illustrates. The most developed embodiment presently available in the market is a method in accordance with FI Patent 77279 and an apparatus realizing said method. It is characteristic of the method in accordance with said patent that the fiber suspension is subjected to axial forces varying in strength and effective direction, the direction and strength of which depend on the axial position between the effective point and the counter surface of the screen cylinder, and which are used to change the axial speed profile of the fiber suspension yet maintaining the direction of the flow continuously towards the discharge end.
Better screening results are obtained with the above described apparatus than the previous prior art apparatuses, in Other words capacity, clarity, etc. according to the need. However, said apparatus is also liable to problems, when it is tended to utilize the most of the characteristics thereof.
The present invention relates to a pulse member of a screener, which has the characteristic feature of subjecting the screen surface to a pulse clearing said screen surface without allowing the pressure effect "slip away" past the point being cleaned. It is also characteristic of the pulse member in accordance with the present invention that the pulse clearing the screen surface is the stronger the thicker the matting is or the worse the screen surface is clogged.
It is characteristic of an embodiment of the apparatus in accordance with the present invention being provided with a pulse member with a continuous surface having a leading edge and a trailing edge facing the screen cylinder, and of which screen cylinder and pulse member at least one is rotatably mounted to the shaft that said at least one continuous surface facing said screen cylinder is provided with at least two substantially circumferentially extending channels having a cross-sectional area widening towards said trailing edge by means of which channels said surface of the screen cylinder is subjected to a strong suction effect in order to maintain said screen surface clean.
It is also characteristic of a second embodiment of the apparatus in accordance with the present invention being provided with a pulse member with a continuous surface being provided with at least one element having a leading surface and a trailing surface that said element is provided with at least one substantially circumferentially extending channel having a cross-sectional area widening along said trailing surface away from said leading surface by means of which channels the surface of the screen cylinder is subjected to a strong suction effect in order to maintain said surface clean.